Surge arresters are used to protect expensive electrical equipment in power systems from overvoltages. A common product for this purpose is cylindrical blocks of metal oxide, for example zinc oxide, so-called varistors. These have the property that the resistance is high at low voltage but low at high voltage.
When the operating voltage is higher than one single varistor can resist and still exhibit a high resistance, several varistor blocks are connected in series in a stack. To carry large currents through a stack, a sufficient contact pressure must be achieved between the blocks. This is achieved by providing clamping members, which connect first and second end electrodes, and a length adjustment device provided between the first electrode and the stack of varistors, which ensures that the clamping members exert sufficient pressure on the varistor blocks.
To achieve improved resistance to transversal mechanical influence, a central pivot member is placed between the lower end electrode and the nearest zinc-oxide block in the stack.
An example of a prior art surge arrester is disclosed in the European patent publication EP1625600 A1, which is incorporated herein by reference.